API TestingREST, GraphQL, SOAP validated before your frontend team touches them.

Functional correctness is the starting point, not the finish. We validate status codes and payloads against schema, then push on the parts teams usually skip: authentication and authorisation boundaries, including whether one tenant's token can reach another tenant's data; error handling and the shape of failure responses; rate limits and throttling behaviour; idempotency on retries; pagination and sorting edge cases; and backward compatibility when a version is bumped.

For GraphQL we add query-depth and complexity limits, field-level authorisation, and N+1 behaviour under realistic queries. For legacy SOAP estates we validate against WSDL and handle the fault contracts properly. Tooling is Postman and Karate for functional and contract work, with JMeter or k6 when API performance is in scope.

Where services are owned by different teams, consumer-driven contract Testing stops integration breaking silently. Each consumer declares what it needs; each provider verifies it still delivers that on every build. The breakage surfaces in the provider's pipeline, with the provider's team, at the moment they introduce it, rather than in a shared environment two weeks later. We wrote up the practical version of this in Shift-Left API Contract Testing.